Disclosed herein is a method of producing photovoltaic quality amorphous alloys which are characterized by a low density of defect states in the energy gaps thereof and good electrical, optical and mechanical properties. As used herein, "density of defect states" is defined as the number of defect states, or deep electronic trap sites such as dangling bonds, per unit volume in the band gap of a given material.
Amorphous alloys, such as amorphous silicon alloys and amorphous germanium alloys, have previously been produced by glow discharge plasma deposition, chemical vapor deposition, evaporation and sputtering processes. While these processes will be referred to in greater detail hereinafter, it is sufficient for purposes of the instant discussion to note that the amorphous alloy materials so produced have a relatively high density of defect states in the energy gaps thereof. Since a significant industrial use of the amorphous silicon and germanium alloys is in the production of semiconductor devices such as photovoltaic cells, it is important that the alloys used to fabricate the photovoltaic cells provide, inter alia, good electrical transport properties. However, amorphous alloys having a high density of defect states in the energy gaps thereof are characterized by a large number of deep electronic trap sites which result in low drift mobility and short lifetimes for the recombination of free carriers. Amorphous semiconductors are extremely attractive alternatives to crystalline semiconductors because they (1) can be produced as large area devices, and (2) possess a band gap with a high value for the optical absorption coefficient as compared to the direct band gap of crystalline materials. It is therefore extremely important for the density of defect states at the Fermi level of the amorphous semiconductor alloys to be reduced in order for amorphous semiconductors to compete in a cost effective manner with their crystalline counterparts.
Whereas it now possible to provide the wider band gap amorphous alloys with an acceptably low, although not optimized, density of defect states, it is the narrow band gap alloys (about 1.2eV and below) which have yet to be produced with a sufficiently low density of defect states to make photovoltaic quality films therefrom. It is believed the reason photovoltaic quality low band gap alloys have not as yet been produced is because (1) the environment in which these low band gap amorphous materials have been deposited was contaminated by a great variety of unwanted species and impurities and (2) the previous methods of combining the precursor materials were not effected so as to optimize tetrahedral coordination of the deposited amorphous alloy. The numerous impurities produced by, as well as the parameters involved with, plasma deposition, evaporation, sputtering etc. are inherently uncontrollable, and therefore these processes are unable to provide the extremely high efficiency 30% cells which are theoretically possible.
Since the lowest energy state of amorphous solids such as silicon and germanium occurs if those solids recombine in crystalline form, the aforementioned methods of depositing amorphous alloys resulted in the deposition of a low density, porous amorphous material that contained a great number of vacancies and voids. In order to provide a high, photovoltaic quality amorphous alloy, it is therefore necessary to modify known methods of deposition so as to either (1) reduce the number of vacancies and voids, or (2) make these vacancies and voids become a helpful part of the process. Only by improving the number of vacancies and voids can a high density, low defect material characterized by good transport properties be produced.
Before proceeding further, note that amorphicity is a generic term referring only to a lack of long range periodicity. In order to understand amorphous materials, the stoichiometry of the material, the type of chemical bonding, the number of bonds generated by the local order (its coordination), and the influence of the entire local environment upon the resulting varied configurations contained in the amorphous solid must be considered. Amorphous materials, rather than being viewed as a random packing of atoms characterized as hard spheres, should be thought of as composed of an interactive matrix influenced by electronic configurations. If, however, one is able to outwit the normal relaxations of the amorphous material and utilize the available three-dimensional freedom of the amorphous state, entirely new amorphous materials may be fabricated. Of course, this creation of new amorphous materials requires the use of new processing techniques.
The inescapable conclusion to be derived from the foregoing is that by properly coordinating process parameters, it should be possible to create deviant, but desirable, electronic configurations of amorphous materials. In order to understand this concept, it must be realized that amorphous materials have several different bonding configurations as available energetic options. For instance, elemental amorphous silicon, although normally tetrahedrally coordinated, has some atoms which are not tetrahedrally bonded. Local order is always specific and coexists in several configurations in every amorphous material. Steric and isomeric considerations are involved both with factors influencing amorphicity and with those creating defects in the materials. The constraints in amorphous materials are involved with assymetrical, spatial and energetic relationships of atoms permitted by the varying three dimensional chemical and geometrical possibilities afforded by an amorphous solid. In such a solid there is not only a spectrum of bonding which spans from metallic to ionic, but a spectrum of bond strengths. A major factor involved in the spectrum of bond strengths is the competitive force of the chemical environment which acts to influence and alter the bond energy. Based on the foregoing, it should be apparent that a greater number of weaker bonds are presented in an amorphous material than in its crystalline counterpart.
There exists an important energetic process that leads a material toward amorphicity. More specifically, the preferred chemical bonding of atoms and the field produced by nonbonding electrons can give a molecular structure a proclivity toward an anticrystalline state. The geometries or shapes of these structures are complex, distorted ones formed by localized pressures, repulsions and attractions of surrounding forces. These forces cause compression in one area, elongation in a second, twisting in a third, all in contradistinction to the perfectly repetitive crystalline cell. These tangled networks also include crosslinks and bridging atoms and are constrained by electron orbital relationships including (1) lone or inert pair electrons, and (2) the chemical influence, mechanical presence and the spatial relationships of nearest neighbors. It can therefore be appreciated that the energetic considerations necessary to complete coordination depend upon the ability to spatially and energetically mate bonding positions. This is the reason why elemental silicon (being tetrahedrally coordinated) has a greater number of dangling bonds, weakened bonds and voids than its crystalline counterparts. This gives rise to the concept of utilizing other elements such as hydrogen and fluorine to passivate those defects. While silicon:hydrogen alloys possess more complete bonding units, the substitution or addition of fluorine for hydrogen will provide the amorphous alloy with greater stability and improved electrical properties.
In amorphous alloys the normal equilibrium bonding can be disturbed by creating new configurations through the addition of a compensating element or elements with multi-orbital possibilites. For instance, alloying permits the optical band gap to be tailored for specific applications and yet permits chemical modification or doping by affecting defect states in the gap, thereby controlling electrical conduction. Alloys can be made by using many possible elements, including multi-orbital modifiers such as d-band elements. The orbitals of such elements can enter the bonding matrix, but, more importantly, can eliminate or create states in the energy gap. Therefore, the elimination of the crystalline constraints permits a greater variety of bonding and antibonding orbital relationships than are present in a crystalline solid and represents the key in the synthesization of new amorphous materials.
In summation, the transport properties of amorphous materials are characterized by deviant bonding directly involved with fluctuations of coordination and directionality of the bonding and nonbonding orbitals. The interaction of these spatially and energetically varied orbitals in three-dimentional space provides the opportunity for unusual electronic excitation and recombination mechanisms. It is the chemical and structural basis of amorphous materials that permits the utilization of the "super halogenicity" of fluorine for its organizing and expanding influence to manipulate size and charge of various atoms, and to design atomic and molecular configurations best suited for specific purposes.
In other words, it is possible to synthesize and independently control all relevant characteristics of amorphous alloys such as optical band gap, electrical activation energy, melting temperature, hopping conduction, and even thermal conductivity if the proper manufacturing techniques are employed. Of course the implementation of proper manufacturing techniques requires a departure from convention. However, it is just such novel processing technology for producing up to 30% efficient photovoltaic quality amorphous material with optimum synthesized characteristics to which the instant application is directed.
The ultrahigh vacuum deposition and diffusion process of the present invention, due to the contamination controllability which it affords, provides a means of producing photovoltaic quality narrow band gap amorphous alloys. Obviously, the process of the present invention may also be employed to form photovoltaic quality wider band gap amorphous alloys, such as silicon alloys. Moreover, in those instances in which defect states in the energy gap of amorphous silicon alloys are the result of undercoordination, the production techniques described herein can be employed to reduce the density of those defect states. Utilization of the ultrahigh vacuum deposition and diffusion process described herein for passivating defect states is of special importance when silicon is employed as an alloying agent in combination with a narrow band gap material such as germanium which has a tendency to become divalent, or trivalent, or assume other nontetrahedral configurations. Included within the scope of the present invention is the incorporation of the amorphous materials and compensating or density of states reducing elements in discrete layers as the amorphous alloy is deposited onto a substrate so as to provide further control of the electrical, chemical and physical properties of the deposited alloy. Amorphous alloys produced in accordance with the concepts embodied by the present invention result in the fabrication of photovoltaic quality materials characterized by a low density of defect states in the energy gaps thereof, and a high degree of local order.
Silicon is the basis of the huge crystalline semiconductor industry and is the material which has produced expensive high efficiency (18 percent) crystalline solar cells. When crystalline semiconductor technology became commercially viable, it initiated the present semiconductor device manufacturing industry. This was due to the ability of the scientist to grow substantially defect-free germanium and particularly defect-free silicon crystals, and then turn those crystalline materials into extrinsic materials with p-type and n-type conductivity regions therein. This was accomplished by diffusing parts per million of donor (n) or acceptor (p) dopant materials introduced as substitutional impurities into the substantially pure crystalline materials, to increase their electrical conductivity and to control their p or n conduction type.
These crystal growing processes produce such relatively small crystals that solar cells require the assembly of many single crystals to encompass the desired area of only a single solar cell panel. The amount of energy necessary to make a solar cell in this process, the restrictions imposed by the size limitations of the silicon crystal, and the necessity to cut up and assemble such crystalline material have all resulted in an impossible economic barrier to the large scale use of crystalline semiconductor solar cells for energy conversion. Further, crystalline silicon has an indirect optical edge which results in poor light absorption in the material. Because of the poor light absorption crystalline solar cells have to be at least 50 microns thick to acceptably absorb the incident sunlight. Even if the single crystal material is replaced by polycrystalline silicon which can be produced by cheaper production processes, the indirect optical edge is still maintained; hence the material thickness is not reduced. The polycrystalline material also involves the addition of grain boundaries and other defect problems.
In summary, crystal silicon devices (1) have fixed parameters which are not variable as desired, (2) require large amounts of material, (3) are only producible in relatively small areas and (4) are expensive and time consuming to produce. Devices manufactured with amorphous alloys can eliminate these crystalline silicon disadvantages. Amorphous silicon alloys have an optical absorption edge having properties similar to a direct gap semiconductor and only a material thickness of one micron or less is necessary to absorb the same amount of sunlight as the 50 micron thick crystalline silicon. Further, amorphous alloys can be made faster, easier and in larger areas than can crystal silicon alloys, thereby reducing assembly time and cost.
Accordingly, a considerable effort has been made to develop processes for readily depositing amorphous semiconductor alloys or films, each of which can encompass relatively large areas, limited only by the size of the deposition equipment, and which can be readily doped to form p-type and n-type materials where p-n junction devices are to be made therefrom equivalent to those produced by their crystalline counterparts. For many years such work was substantially unproductive. Amorphous silicon or germanium (Group IV) films are normally four-fold coordinated and were found to have microvoids and dangling bonds and other defects which produce a high density of localized states in the energy gap thereof. The presence of a high density of localized states in the energy gap of amorphous semiconductor films results in a low degree of photoconductivity and short carrier lifetime, making such films unsuitable for photoresponsive applications. Additionally, such films cannot be successfully doped or otherwise modified to shift the Fermi level close to the conduction or valence bands, making them unsuitable for making p-n junctions for solar cell and current control device applications.
In an attempt to minimize the aforementioned problems involved with amorphous silicon and germanium, W. E. Spear and P. G. LeComber of Carnegie Laboratory of Physics, University of Dundee, in Dundee, Scotland, did some work on "Substitutional Doping of Amorphous Silicon", as reported in a paper published in Solid State Communications, Vol. 17, pp 1193-1196,1975, toward the end of reducing the localized states in the energy gap in amorphous silicon or germanium to make the same approximate more closely intrinsic crystalline silicon or germanium, and of substitutionally doping the amorphous materials with suitable classic dopants, as in doping crystalline materials, to make them extrinsic and of p or n conduction types.
The reduction of the localized states was accomplished by glow discharge deposition of amorphous silicon alloy films wherein a gas of silane (SiH.sub.4) was passed through a reaction tube for decomposition by an r.f. glow discharge and deposition onto a substrate at a substrate temperature of about 300.degree.-600.degree. K. (27-327.degree. C.). The material so deposited on the substrate was an intrinsic amorphous material consisting of silicon and hydrogen. To produce a doped amorphous material, a phosphine (PH.sub.3) for n-type conduction or diborane (B.sub.2 H.sub.6) for p-type conduction were premixed with the silane gas and passed through the glow discharge reaction tube under the same operating conditions. The gaseous concentration of the dopants used was between about 5.times.10.sup.-6 and 10.sup.-2 parts per volume. The material so deposited including supposedly substitutional phosphorus or boron dopant and was shown to be extrinsic and of n or p conduction type.
While it was not known by these researchers, it is now known by the work of others that the hydrogen in the silane combines at an optimum temperature with many of the dangling bonds of the silicon during the glow discharge deposition, to substantially reduce the density of the localized states in the energy gap toward the end of making the amorphous material approximate more nearly the corresponding crystalline material.
D. I. Jones, W. E. Spear, P. G. LeComber, S. Li, and R. Martins also worked on preparing a-Ge:H from GeH.sub.4 using similar glow discharge plasma deposition techniques. However, the material obtained gave evidence of a high density of localized states in the energy gap thereof. Although the material could be doped, the efficiency was substantially reduced from that obtainable with a-Si:H. In this work reported in Philsophical Magazine B, Vol. 39, p. 147 (1979) the authors conclude that because of the large density of gap states the material obtained is ". . . a less attractive material than a-Si for doping experiments and possible applications."
After developing a successful process for the glow discharge deposition of silicon from silane gas, research was conducted to develop a process for sputter depositing amorphous silicon films in an atmosphere of argon (required by the sputtering deposition process) and molecular hydrogen, to determine the results of such molecular hydrogen on the characteristics of the deposited amorphous silicon film. This research indicated that the molecular hydrogen acted as an altering agent which bonded in such a way as to reduce the localized defect states in the energy gap. However, the reduction of states in the energy gap achieved by the sputter deposition process was much less than that achieved by the silane deposition process described above. The p and n dopant gases (previously detailed with respect to the silane deposition process) also were introduced in the sputtering process to produce p and n doped materials. The resultant materials possessed a lower doping efficiency than the materials produced in the glow discharge process. However, neither the vapor deposition nor the sputtering deposition techniques of depositing amorphous semiconductor layers provided n-doped and p-doped materials with sufficiently high acceptor concentration to produce commercial p-n or p-i-n junction devices. While the n-doping efficiency was below acceptable commercial levels, the p-doping efficiency was particularly unacceptable since the width of the band gap was reduced and the number of localized states in the band gap was increased.
Further research was conducted to additionally reduce defect states in amorphous silicon alloys or in amorphous silicon:hydrogen alloys. Fluorine was found to readily diffuse into and bond to the amorphous silicon, substantially reducing the density of localized states therein, because of its reactivity occasioned by the small size of the fluorine atoms which enables them to be readily introduced into the amorphous silicon matrix. Fluorine was found to bond to the dangling bonds of the silicon in a manner which is more stable and efficient than is possible when hydrogen is used. However, fluorine introduced into amorphous germanium alloys or amorphous germanium:hydrogen alloys, has not, up to the date of the instant invention, produced an optimized narrow band gap amorphous material.
Numerous attempts have been made to construct both natural and synthetic crystalline analogue materials by special layering techniques with the aim of extending the range of desirable properties which were heretofore limited by the availability of natural crystalline materials. One such attempt involved compositional modulation by molecular beam epitaxy (MBE) deposition on single crystal substrates. Esaki, Ludeke and Tsu, in U.S. Pat. No. 3,626,257, describe the fabrication of monolayer semiconductors by one MBE technique. These modulated prior art structures are typically called "superlattices". Superlattice fabrication techniques are based on the concept that layers of materials may be made to form a one-dimensional periodic potential by periodic variation of (1) alloy composition or (2) impurity density. Typically, the largest period in these superlattices is on the order of a few hundred Angstroms, however, monatomic layered superlattice structures have also been constructed. The superlattices can be characterized by the format of (1) several layers of a material "A" (such as GaAs), followed by (2) several layers of a material "B" (such as AlAs), in a repetitive manner, (3) formed on a single crystal substrate. The optimum superlattice is a single crystal synthetic material with good crystalline quality and electron mean free paths greater than the period. Conventional superlattice concepts have been utilized for special electronic and optical effects. The most severe limitation on good quality superlattice fabrication is that the lattice constants must be very carefully matched, thereby limiting the utilization of these superlattices.
In related work, Dingle, et al., see U.S. Pat. No. 4,261,771, disclose quasi-superlattices and non-superlattice structures. The former are comprised of epitaxially grown crystalline islands of a foreign material in an otherwise homogeneous layered background material. The latter, non-superlattice structures, are an extension of quasi-superlattice materials in that the islands are grown into columns which extend vertically through the homogeneous layered background material.
In addition to MBE superlattice construction techniques, other researchers have developed layered synthetic microstructures which utilize different forms of vapor deposition, including diode sputtering, magnetron sputtering, and standard multisource evaporation and organo-metallic vapor deposition.
The layer dimensions of these synthetically produced materials may be controlled by (1) shutters, (2) moving the substrates relative to the sources of material, (3) control of the partial pressure of the reactive gases, or (4) combinations of shutters and substrate movement. The resultant materials have been formed from crystalline layers, noncrystalline layers and mixtures thereof. However, each of the research efforts so far reported has attempted to synthesize superlattice-type structures by precisely reproducing the deposition conditions on a periodically reoccuring basis. These materials can be thought of as synthetic crystals or crystal analogues in which the long range periodicity, the repetition of a particular combination of layers, or the grading of layer spacing must be closely maintained. Consequently, superlattice structures, so constructed, are both structurally and chemically homogeneous in the x-y plane, but periodic in the z direction.
In addition to the synthetic material-producing techniques described above, compositionally varied materials and processes for their production are disclosed in copending U.S. patent application Ser. No. 422,155, filed Sept. 23, 1982, entitled Compositionally Varied Materials And Method For Synthesizing The Materials, by Stanford R. Ovshinsky, assigned to the assignee of the instant application and which is incorporated herein by reference.
Other methods of producing amorphous alloy materials, specially adapted for photovoltaic applications, are disclosed in Assignee's U.S. Pat. Nos. 4,217,374; 4,226,898 and 4,342,004. The deposition techniques described therein are adapted to produce alloy materials including germanium, tin, fluorine and hydrogen as well as silicon. The alloy materials are produced by vapor and plasma activated deposition processes. Further, (1) amorphous cascade type multiple cell devices are disclosed in copending U.S. patent application Ser. No. 427,757, entitled Multiple Cell Photoresponsive Amorphous Alloys and Devices, and (2) amorphous germanium alloys are disclosed in copending U.S. patent application Ser. No. 427,754, entitled Method Of Making Photoresponsive Amorphous Germanium Alloys And Devices.
Of special significance relative to the concepts embodied in the present application is copending U.S. patent application Ser. No. 514,688 of Stanford R. Ovshinsky filed July 18, 1983 entitled Enhanced Narrow Band Gap Alloys For Photovoltaic Applications, and also assigned to the assignee of the instant applications (hereinafter referred to as "said previous patent application").
Said previous patent application identified the failure of fluorine and hydrogen to compensate for the dangling bonds of narrow band gap amorphous materials as being directly associated with the tendency of narrow band gap amorphous materials, such as germanium, to become divalent or assume other nontetrahedral configurations. Said previous patent application proposed to solve the problem by causing the amorphous narrow band gap materials to bond in a more tetrahedral configuration. More particularly, that invention proposed to minimize or eliminate the tendency of such narrow band gap amorphous materials to assume divalent, distorted tetrahedral, or other nontetrahedral coordination.
Said previous patent application sought to solve the problem of nontetrahedral bonding by promoting or activating the inert pair of valence electrons so as to expand the coordination thereof, which permits their use in bonding with compensating elements. As the coordination of the inert pair of valence electrons was expanded, the production of low band gap materials having the prerequisite low density of defect states was made possible.
The procedure prescribed in said previous patent application included the following steps. First the competing reactions which interfered with or prohibited the activation of the inert pair of valence electrons were eliminated. Second, a density of states reducing element(s) such as hydrogen and/or fluorine was provided in a form which would compensate the low band gap amorphous alloys. If hydrogen, for example, was selected as the density of states reducing element, it would be admixed with the amorphous material in atomic form. Molecular hydrogen was described as not being able to expand the coordination of amorphous materials such as germanium. If fluorine was selected as the density of states reducing element, it would also be provided in free radical form for enhancing formation of the amorphous alloy. Fluorine was noted to be "the super halogen" (the most electronegative element) when supplied in free radical form, and was described as capable of promoting or activating the inert pair of valence electrons by expanding the orbitals, thereby achieving tetrahedral configuration. Fluorine further (1) eliminates dangling bonds, (2) provides ionicity to the bonding to help relieve stresses in the alloyed materials, and (3) acts as a bridging or crosslinking structural element in the expanded structural configurations.
The third necessary step of the procedure described in said previous patent application was to provide external excitation of the density of states reducing element so as to promote the proper tetrahedral bonding. The amorphous material could also be externally excitable so that the energy of the inert pairs of valence electrons thereof are raised, thereby increasing their reactivity. Alternatively or concomitantly, the energy of the substrate and the amorphous alloy being deposited thereon can be raised to the activation level necessary for promoting the orbitals and exciting the electrons to ultimately increase the reactivity of the inert pair of valence electrons.
Said previous patent application described an enhanced narrow band gap amorphous semiconductor alloy and the method for producing same. More particularly, said previous patent application provided a general discourse of and background information for the specific concepts detailed and expanded upon herein. However, the process described therein was primarily directed to codeposition of free radicals of the amorphous material and free radicals of the density of states reducing element in a contaminant-free environment so as to achieve tetrahedral coordination therebetween. The codeposition did not take into account the fact that the resultant amorphous alloy is a heterogeneous material made up of two series of regions, as fully explained hereinafter, which must be individually treated in order to reduce the density of defect states to the lowest possible levels. In contradistinction thereto, the present application is directed to a novel post deposition diffusion process in which unadulterated amorphous material is deposited onto a substrate at a low substrate temperature before the, preferably activated, density of states reducing element is introduced into the ultrahigh vacuum environment. Since (1) an environment is provided in which there are no contaminants present to occupy the available bonding sites of the amorphous material, and (2) the deposition occurs at a low temperature, the deposited amorphous material is a porous mass of voids, vacancies, dangling bonds, etc. Therefore, the density of states reducing element readily diffuses into and is greedily accepted by the reactive amorphous material to reduce the density of defect states in those regions of the heterogeneous amorphous alloy characterized by a relatively low density of defect states. An annealing step completes diffusion of the density of states reducing element through the amorphous material for reducing the density of defect states in those regions of the heterogeneous amorphous alloy characterized by a relatively high density of defect states. In this manner the present invention deals with the heterogeneous nature of the amorphous alloy as never contemplated in prior art methodology. Finally, a strain relieving element may be introduced to relax bonding stresses in the resultant alloy and ion implantation may be employed to reduce the density of defect states in regions of the amorphous alloy which are unaccessible to the diffusion process.
In order to explain the manner in which the basic concepts employed by the present invention solve the problem of forming a photovoltaic quality, tetrahedrally coordinated amorphous material, reference is now made to FIGS. 1-4 of the drawings. FIG. 1 depicts in highly stylized schematic form, an enlarged portion of unadultered amorphous material 9 which was deposited at relatively high (180.degree. to 300.degree. C.) temperature. The enlarged portion of amorphous material 9 is characterized by (1) a continuous random network region 13 characterized by a relatively low density of defect states, hereinafter referred to as a "low defect region", and (2) a region 15 characterized by a relatively high density of defect states, hereinafter referred to as a "high defect region".
As described hereinabove, in a perfect crystalline solid each atom has all of its available bonding sites filled so that the matrix of atoms so formed is at its lowest possible energy level. In contrast thereto, in an amorphous solid the atoms are randomly ordered, i.e., there is no repetitive long range periodic order or pattern of atoms. It therefore becomes very difficult, if not impossible, to satisfy all valency requirements of amorphous solids, since to do so would require bending, stretching, and/or twisting of its chemical bonds. Accordingly, amorphous solids possess a finite number of defect states such as missing, broken, stretched, twisted or highly strained chemical bonds.
It must both be noted herewith and strongly emphasized that the depictions of the low and high defect regions in FIGS. 1, described hereinabove, and in FIGS. 2-4, described hereinafter, are highly stylistic so as to better facilitate the description and understanding of the principles of the instant invention. In actuality, the low defect regions 13 and the high defect regions 15, rather than being of homogeneous size, shape, location and distribution as shown in the figures, are of highly irregular, non-homogeneous size, shape, location and distribution. Specifically, the low defect regions are, in reality, irregularly shaped clusters separated by irregularly spaced, irregularly shaped neighboring clusters. Therefore the accompanying figures are not pictorial illustrations but, rather, are diagrammatic representations intended to symbolically depict the size, shape, proximity of nearest neighbors and geometric distribution of the two series of regions.
Keeping this provision in mind and again referring to FIG. 1, because of the relatively large number of defects such as dangling bonds, trap sites and spin defects, which are present in the high defect regions 15, the amorphous material is in an activated state wherein it greedily awaits the introduction of a passivating agent (a defect state reducing element) such as hydrogen, fluorine, oxygen and the like to complete its coordination and relieve internal bonding stresses. In contradistinction thereto, the amorphous material in the low defect regions 13, has a relatively small number of defects, such as dangling bonds, etc., and is consequently relatively unreactive with and impermeable to said passivating agents. Obviously, the higher the energy level of the valence electrons of the unfilled orbitals of the amorphous material and the valence electrons of the passivating agent, the greater the likelihood of chemical combination therebetween. It is the object of the present invention to entirely diffuse the density of states reducing element into and through both the low defect regions and the high defect regions which combine to form the heterogeneous amorphous alloy. Because the amorphous material is indeed heterogeneous, different techniques must be employed to diffuse a passivating element into the interior confines of that host material. To illustrate the difficulties of diffusing atoms of the passivating element into the interior of the material, note that even the low defect regions contain dangling bonds, electron trap sites, spin defects, etc., said defects in the low defect regions will hereinafter be referred to as "low defect region defects" and stylistically represented by the reference numeral 17. As with the representations of the high defect regions 15 and the low defect regions 13, the low defect region defects 17 are actually of highly irregular shapes, sizes and geometrical distributions. The four, five or six X's depicted in the drawings as symbolizing these low defect region defects 17 are intended to be merely symbolic of the fact that the low defect regions 13, themselves, are not perfectly coordinated.
If the amorphous material 9 was a perfectly arranged cluster of tetrahedrally coordinated atoms, the density of states reducing element would have absolutely no difficulty in diffusing into and through the entire body thereof. However, because even the low defect regions 13 contain defects 17, there are no clearly defined, unobstructed passageways which atoms of the density of states reducing element can follow to passivate dangling bonds, etc. which exist centrally of the body of amorphous material 9. It should be obvious, however, that the atoms of the density of states reducing element can most easily traverse the tortuous path of travel through the low defect regions 13 before reaching and being blocked by the tangled mass of defects that exist in the high defect regions 17.
From the foregoing description, it should be apparent that, in contradistinction to prior art techniques for passivating defect states in the band gap of amorphous materials which treated the amorphous material as being homogeneous, the method proposed herein for reducing those defect states accepts the amorphous material as a heterogeneous body and treats the defect states in the different series of regions, i.e., the series of high defect regions 15 and the series of low defect regions 13, individually. The result is a two pronged process which includes a low temperature diffusion step to passivate states 17 in the low defect regions 13 and and annealing step to passivate states in high defect regions 15, whereby the two pronged process achieves a clear reduction of defect states in the band gap of that amorphous material.
Turning now to FIG. 2, a stylized depiction of an unadulterated, relatively porous, void and vacancy-filled amorphous solid 9' deposited at a relatively low temperature (-20.degree. to 180.degree. C.) is schematically illustrated. As with the amorphous solid 9 shown in FIG. 1, this amorphous solid 9' also includes a series of low defect regions 13' having defect states 17' therein and a series of high defect regions 15'. Comparing the two series of regions which combine to form the heterogeneous network of atoms of amorphous material 9 shown in FIG. 1 with the two series of regions shown in FIG. 2, the morphological differences therebetween are immediately apparent. In FIG. 1 the low defect regions 13 are bounded by relatively narrow high defect regions 15; or, in other words, the low defect regions 13 are situated in close proximity to nearest neighbor clusters of low defect regions 13. In FIG. 2, the low defect regions 13' are bounded by relatively wide high defect regions 15'; or, in other words, the nearest neighbor clusters of low defect regions 13' are not in as close proximity to one another. The foregoing discussion is meant to illustrate the relative ease or difficulty atoms of the density of states reducing element, such as hydrogen or fluorine, would have in diffusing through the continuous random network of high and low defect regions to ultimately diffuse completely into and combine with most of the interior atoms of the amorphous material. Specifically, in the amorphous material 9 of FIG. 1, the relatively narrow high defect regions 15 make it relatively difficult for a fluorine or hydrogen atom to diffuse into and passivate defects located interiorly of the periphery of the material. In contradistinction thereto, the amorphous material 9' shown in FIG. 2 may be readily passivated by fluorine or hydrogen atoms since the relatively wide high defect regions 15' allow for the atoms to diffuse into and combine with the interiorly disposed electrons in the unfilled orbitals of the amorphous material. It should therefore be readily apparent the defect states in amorphous materials prepared using low temperature deposition processes, being porous, void-filled masses, may be more easily passivated than those in the dense material produced using a high temperature deposition process. Of course, the amorphous material 9', itself, contains more high defect regions 15', but those regions can be passivated by the density of states reducing element to achieve better material than can be deposited by the high temperature process, but which cannot be effectively passivated.
FIG. 3 is yet another diagrammatic representation, albeit a more realistic representation of an amorphous solid 9" produced in a low temperature deposition process. The amorphous material 9" includes the series of low defect regions 13" and the series of high defect regions 15" which combine to form the heterogenous network of atoms, as previously described. However, the relative disposition of these regions to their nearest neighbor more candidly depicts the real world. More specifically, the spacing between low defect regions 13" is generally irregular, following no set pattern. Likewise, in reality, the high defect regions 15" vary in size, location, geometric distribution and shape. For example, some high defect regions such as 15a" and 15b" are relatively wide, with nearest low defect region neighbors 15" being spaced a relatively great distance therefrom. On the other hand, other high defect regions such as 15c" and 15d" are relatively narrow, actually approximating regions resulting from the high temperature deposition process of FIG. 1, and having nearest low defect region neighbors 15" being spaced a relatively small distance therefrom. Still other defect regions such as 15c" are relatively wide, but are effectively "walled off" and held relatively incommunicado from the infusing density of states reducing elements by the contiguous relatively narrow high defect regions 15". The diffusion of density of states reducing elements, such as fluorine and hydrogen, through the amorphous material 9" to passivate defect states in the high defect regions 15", thereof will proceed (1) through the low defect regions 13" (passivating defect states 17" therein), and then (2) in those readily accessible and relatively wide, high defect regions such as 15a" and 15b". However, in those high defect regions 15" which are relatively narrow, such as 15d" and 15c", or those regions which are relatively inaccessible, such as 15e", passivation will be more difficult since permeablity of the fluorine or hydrogen atoms is prohibited by the narrow high defect regions.
One possible solution to the problem of passivating the relatively inaccessible high defect regions (regions even inaccessible after annealing) is addressed in FIG. 4, which is again a schematic representation, substantially identical to the representation of the series of defect regions illustrated in FIG. 3. According to this solution, a passivating agent, such as fluorine or hydrogen, may be energetically directed through the amorphous solid 9" from a source such as an ion implantation gun 21. Rather than relying upon diffusion which can be blocked by the narrow high defect regions, such as 15c" and 15d," energetic fluorine or hydrogen ions are implanted into the inaccessible high defect regions 15", of the amorphous material 9", thereby reducing the density of defect states interiorly thereof. Obviously, the relatively inaccessible high defect regions can also be passivated by other techniques such as ultrasonic agitation.
In summary, prior to the instant application, with the exception of said previous patent application, efforts have been made to provide synthesized amorphous alloy materials having specific desirable properties. The resultant materials have, in some cases, been provided with desirable chemical, electrical and optical characteristics when employed in tandem or single cell photovoltaic and similar semiconductor devices. However, no process has treated amorphous material as heterogeneous nor been able to fabricate a narrow band gap alloy material which has optimum chemical, electrical and optical properties for utilization as the bottom cell in a multiple cell photovoltaic device which makes use of low temperature evaporation methodology. The amorphous germanium alloys which have been reported to date (with the exception of said previous patent application) possess unacceptable levels of defect states in the band gaps thereof so that the actual technology required to produce 30% cell efficiency is still lacking. The previously described conventional methods of producing amorphous alloys by glow discharge plasma deposition, sputtering, evaporation, etc. have as yet to even optimize the removal of defect states in amorphous silicon alloys, and have not produced photovoltaic quality amorphous germanium alloys. In contradistinction to the foregoing techniques and processes for the fabrication of amorphous alloys possessing narrow band gaps in particular, and amorphous alloys regardless of band gap width in general, the invention described herein is specifically directed to a method employing (1) low temperature diffusion for passivating the defect states in the low defect regions of all amorphous alloys, and (2) an annealing step for passivating the defect states in high defect regions so as to provide amorphous semiconductor alloys capable of achieving 30% efficiency when used in photovoltaic devices.